How can I get the first async response fastest (and don't perform the remainder)?

Question

Here's the setup: There is a federal remote service which returns whether a particular value is correct or not correct. We can send requests as we like, up to 50 per request to the remote service.

Since we need to only use the correct value, and the set of possible values is small (~700), we can just send 15 or so batch requests of 50 and the correct value will be part of the result set. As such, I've used the following code:

Observable
    .Range(0, requests.Count)
    .Select(i => Observable.FromAsync(async () =>
    {
        responses.Add(await client.FederalService.VerifyAsync(requests[i]));
        Console.Write(".");
    }))
    .Merge(8)
    .Wait();

But - what I don't like about this is that if one of the earlier requests has the correct value, I still run all the possibilities through the service wasting time. I'm trying to make this run as fast as possible. I know the exit condition (response code is from 1 to 99, any response code within 50-59 indicates the value is "correct").

Is there a way to make this code a little smarter, so we minimize the number of requests? Unfortunately, the value we are verifying is distributed evenly so sorting the requests does nothing (that I'm aware of).

Using System.Reactive, I'm not sure. I don't see the method available for use in that code. — djbyter, Apr 13 '17 at 16:10

VMAtm · Accepted Answer · 2017-04-18T14:39:28.880

3

You should consider usage of the FirstAsync method here:

The secret in our example is the FirstAsync method. We are actually awaiting the first result returned by our observable and don’t care about any further results.

So your code could be like this:

await Observable
    .Range(0, requests.Count)
    .Select(i => Observable.FromAsync(async () =>
    {
        responses.Add(await client.FederalService.VerifyAsync(requests[i]));
        Console.Write(".");
    }))
    .FirstAsync()
    .Subscribe(Console.WriteLine);

> System.Reactive.Linq.ObservableImpl.Defer`1[System.Reactive.Unit]

Rx and Await: Some Notes article provides some tricks with similar methods. For example, you have an overload for FirstAsync, which can be filtered, as the LINQ' method First:

await Observable
    .Range(0, requests.Count)
    .Select(i => Observable.FromAsync(async () =>
    {
        responses.Add(await client.FederalService.VerifyAsync(requests[i]));
        Console.Write(".");
    }))
    .FirstAsync(r => /* do the check here */)
    .Subscribe(Console.WriteLine);

edited Apr 18 '17 at 14:39

answered Apr 13 '17 at 23:00

VMAtm

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FirstAsync doesn't seem to allow me to check the condition to see if I have the result. – djbyter Apr 18 '17 at 13:39
@djbyter `FirstAsync` is similar to `First` in `LINQ`, did you try to filter your events? – VMAtm Apr 18 '17 at 14:39
This did the trick. Works well, I just didn't know that FirstAsync was able to take a condition. – djbyter Apr 18 '17 at 15:40
`Rx` team designed these methods similar to `LINQ` :) Good luck with your projects – VMAtm Apr 18 '17 at 15:51

score 2 · Answer 2 · answered Apr 14 '17 at 04:42

You're pretty close. Change your observable to this:

Observable
    .Range(0, requests.Count)
    .Select(i => Observable.FromAsync(async () =>
    {
        var response = await Task.FromResult(i); //replace with client.FederalService.VerifyAsync(requests[i])
        responses.Add(response);
        Console.Write($"{i}.");
        var responseCode = response; //replace with however you get the response code.
        return responseCode >= 50 && responseCode <= 59;
    }))
    .Merge(8)
    .Where(b => b)
    .Take(1)
    .Wait();

This way your observable continues to emit values, so you can continue acting on it.

This seems to work better, as I get the result asap, but it does continue to run all requests. — djbyter, Apr 18 '17 at 14:05

How can I get the first async response fastest (and don't perform the remainder)?

2 Answers2